Electric discharge device



NOV. 28, 1944. A w HULL ELECTRIC DISCHARGE DEVICE Filed March 1, 1943 Fig. I

Inventor: Albert W. Hull, b W 6.

His Attorney.

Patented Nov. 28, 1944 ELECTRIC DISCHARGE DEVICE Albert W. Hull, Schenectady, N. Y., asslgnor to General Electric Company, a corporation of New York Application March 1, 1943, Serial No. 477,533

11 Claims.

The present application is a continuation-inpart of prior application Serial No. 445,144, filed May 30, 1942. a Y

The present invention relates to electric discharge devices which utilize vapor as an ionizing medium.

In the operation of such discharge devices it is important that the pressure of the vapor should be maintained within predetermined limits. At pressures above a permissible maximum, the device may lose its rectifying characteristic if it is of the two-electrode type, or may cease to be controllable if it is of the three-electrode, grid-controlled type. At. pressures below a permissible minimum, on the other hand, the lack of sumcient positive ions to neutralize the electron space charge may cause the voltagedrop to be abnormally high, with resultant injury to the cathode, and may lead to high-voltage surges which tend to break down the electrical insulation. These maximum and minimum values depend on the nature of the device but may be said in general to fall within the range of about one to one hundred microns of mercury pressure. a

The pressure of the vapor filling of a discharge device can be controlled by the provision of a condensing region which is maintained at a temperature below that of the remaining parts of the device. This expedient keeps the pressure existing in the discharge space at the desired value by condensing all vapor inexcess of that required to maintain a pressure corresponding approximately to the temperature of the condensing region.

While regulation of the character just specified is satisfactory after a condition of stable operation has been obtained, it is not entirely adequate to solve the problems presented in connection with restarting a discharge device after a period of disuse. After such disuse, it is frequently found that in a device containing mercury vapor by diifusion or otherwise, mercury films have become more or. less uniformly distributed by condensation on the walls and electrodes in the discharge space. Under these conditions the initial operation of the device will result in abnormally high pressures being produced by rapid vaporization of the condensate from the heated parts. As a result, imperfect or uncontrollable operation may occur and persist for from several minutes to several hours, depending upon the time required to cause all the excess vapor to be condensed at the normal condensation point. It frequently happens also that liquid material runs out of the condensing region and becomes dispersed over the walls because of tilting of the tube during handling with results similar to those just described.

It is an object of the present invention to obviate these difficulties by the provision of means for assuring the retention of .unvaporized material in a selected condensation region during periods of disuse of the discharge device. In accordance with my invention the material to be vaporized is maintained at a predetermined elevated temperature during operation of theelectrical discharge device bya combination of control means whichare described hereinafter.

In a preferred form of my invention, mercury vapor is derived preferably from an amalgam of a class which is solid at 'normaltemperatures (i. e., temperatures encountered during periods of disuse of the device) and which, when unheated, provides a partial mercury pressure too low to permit any substantial diffusion of mercury to other parts of the device. Cadmium and tin amalgams represent suitable embodiments of this aspect of the invention. The vapor may be derived, however, from liquid mercury.

The features desired to be'protected herein are pointed out in the appended claims. The invention itself may best be understood by reference to the following description takn in connection with the drawing, in which Fig. 1 is a longitudinal section of a discharge device suit-l ably embodying the invention; Fig. 2 is a fragmentary section showing on aimagniiled scale the thermostatic control of Fig.1; Fig. 3 is a section on an enlarged scale of another embodiment of electrical means for controlling the temperature of the amalgam; and Figs. 4 and 5 are sectional views of alternative control'means which may be used in place of the one embodied in the devices of Figs. 1 to3.

The construction shown in Fig. lrepreSents a discharge device of a typewhich as to its general structural, features is described in United States Patent 2,292,081, in the name of Harold T. Maser, which is assigned to the samelassignee as the present invention. This device, which is illustrative of devices to which my'invention is applicable, comprises a metal envelope I which may consist of a section of steel tubing. The envelope may be closed at its upper and lower ends by flanged headers indicated at H and I2, respecmounted on a heavy conductiverod II and is insulatingly supported from the header through a glass sleeve ll. The sleeve I1 is hermetically joined to the header by means of a flanged metal cylinder I6 which is sealed to the sleeve and welded to the header. It is closed at its upper extremity by means oi a metal can 26. A flexible conductor 22 is secured to the cap 26 by being soldered into a metal ferrule 23 which is afllxed to the cap.

At the lower end of the envelope there is provided a thermionic cathode 26 having its active parts (not shown) enclosed within a metal heat shield 26. As well known, the internal structure of the cathode includes a filamentary heater and means providing extended emlssive surfaces adapted to be maintained at an elevated temperature by a heater. A cathode of this type is shown and described in United States Patents 2,201,720 and 2,201,721 of Thomas A. Elder. The cathode .26 is supported irom the envelope header |2 by being secured to metal brackets 26 and 26, these brackets also serving as a part of the current supply system for the cathode. The remainder of the cathode heating circuit includes an electric supply conductor 6| which may be assumed to connect inside the cathode structurewith the filamentary heater previously referred to in accordance with well known practice.

In the operation of the device the initiation of a discharge between the cathode 26 and the anode I4 is controlled by means of a grid in the form oi. a conductive cylinder 64. The grid is rigidly mounted with respect to the main body of the envelope by means of a heavy conductor 35 which is insulatingly supported from the envelope through a glass sleeve 66. In addition to the sleeve 66. the supporting structure includes a metal cap 31 and a metal cylinder 68, these elements being hermetically joined to the respective ends of the sleeve. Apertured metal baffles 40, 4| arranged within the discharge envelope on opposite sides of the grid 31 serve to shield the grid and to make its action more constant and reliable.

The operation of the device in its intended manner requires the presence within the discharge space 01a quantity of ionizable vapor, which ordinarily will be mercury vapor. As has been previously stated herein, it is essential that the pressure of the vapor be kept within definite limits in orde to assure proper operation.

These 'diflicultles are overcome in accordance with the present invention by providing as a source of vapor aquantity of easily vaporizable material, most suitably mercury or a suitable amalgam, in a projecting container adapted to be held at a desired elevated temperature and which communicates with the main envelope. In the construction illustrated by Figs. -1 and 2, such container is in the form of a well or tubulation of metal (e. g., steel) 46, which as shown communicates by a wide bore non-constricted open-- tubulation 46 contains a quantity 41 of mercury.

In the illustrative device shown in Fig. 3, the tubulation 46' contains an amalgam.

When it is desired to put the discharge device into operation. the cathode 26 is heated to a desired operating temperature. While some heat is conducted from the cathode through the tubulation 46 to the mercury charge 41, additional heat is required for the establishment of sumcient mercury vapor pressure in the discharge space. For this reason an automatic means is provided for heating the charge 41 of mercury, or amalgam, to an elevated temperature.

In the device shown in Fig. 1 heat is supplied by electric induction. Surrounding the tubulation 46 is an electrical coil or winding 46. It is supplied with alternating current of suitable frequency by the conductors 46, 46' in series with a contactor which is normally closed when the device is operating. The tubulation 46 is prelerably surrounded by an iron tube 6| to increase its inductive coupling to the coil 46. The heating of the tubulation 46 and the iron tube 6| func- -tioning as secondary causes the mercury to be vaporized.

Heat also is imparted to an easily volatilizable material 62 contained in a cuplike receptacle 6! which communicates with the interior or an adjoining expansion chamber or Sylphon bellows 64. The rise of pressure of the vapor of the material 52, which may consist of ether, for example which has a higher vapor pressure than mercury, causes the expansion member to open the electric contacts 66, 65'. These contacts are connected in the circuit 66, 61 of a relay magnet 66 which, when energized, in turn opens the main contacts 60 and deenergizes the primary inductive winding 48. The expansion member 64 may be set or calibrated to maintain by its control of the make-and-break contacts 66, 66 a predetermined vapor pressure within the chamber tric energy is supplied to the conductor 66 from.

a conductor 6| which is connected to the housing l0 by a terminal 62. From here the conductor 56 is led to the contacts 66 through the terminal 63, see Fig. 2. The conductor 51 is connected to the cooperating contact 66' through 1an insulating bushing 64 in the wall of the hous- Electric power is supplied to the conductors 3|, 6| by a transformer (not shown), or other suitable source. Assuming the vaporizable material 41 to consist of an amalgam, such for example as a cadmium amalgam, then if the tubulation were consistently to remain at a temperature somewhat below the temperature of the envelope in as a whole, the mercury vapor pressure actually realized will be a partial pressure determined by the proportion of mercury in the amalgam. This means that for a given operating temperature the mercury vapor pressure will be materially less than in a situation in which the mercury vapor is supplied from unalloyed mercury.

- In other words, a somewhat higher temperature will be necessary to realize a given mercury vapor pressure with the amalgam than with an unamalgamated mercury source. This is an advantageous consideration in that it permits the device to be used in locations where the ambient temperature is high without'the necessity oi. using cooling means to keep the mercury vapor pressure within the desired limits.

Even more important from the standpoint of the present invention, however, is the fact that the location of an amalgam within the condensing tubulation avoids the danger oi the mercury constituent being distilled into the main discharge space during inoperative periods of the device. This is explained by the fact that after a period of operation, as the tube cools, the mercury recondenses into the tubulation it which remains the coolest part of the envelope structure until complete thermal equilibrium is attained. Since the condensed mercury reamalgamates with the cadmium or other associated metal, it is unable (because of insufllcient vapor pressure) to redistill into the main discharge space when the whole tube structure reaches an equilibrium condition at the ambient temperature. When a tube'has been idle for two weeks and then full voltage is applied with only enough preliminary heating of the tube to bring the cathode to a fully operative condition, normal operation of the tube is realized at once.

Another advantage of the use of a solid amalgam in place of liquid mercury is thatit may be retained in the tubulation by suitable means, such as the metal gauze I6 shown in Fig. 3, and will not spill or run into the main envelope when the device is tilted or inverted. Such spilling would necessitate a prolonged heating to drive the mercury back into the tubulation.

A still further advantage which is realized by a provision of the designated arrangement for keeping the mercury in the condensing tubulation during idle periods has to do with the protection of the cathode against deterioration of the activating coating on its emitting surfaces. It frequently has been observed that oxide cathodes used in mercury tubes tend to flake when left idle for long periods. This flaking, which does not occur in high vacuum tubes, is believed .to be caused by absorption of mercury at the oxide-to-metal interface of the cathode. The

use of amalgam reduces this tendency by lowerlllilg the mercury vapor pressure during idle peods.

While cadmium is considered to be a preferred amalgamating substance for the use specified, certain other metals, including specifically tin, may be used alternatively.

A modified arrangement for maintaining: a desired vapor pressure is illustrated in Fig, 3. It may replace the induction heater arrangement shown in Figs. 1 and 2. (The electrodes of the main discharge device may be assumed to be of the same general character as those illustrated in Fig. 1.) The regulating apparatus consists of a case 65 which may be constituted of suitable insulating material, for example, a phenolic condensation product. Flat carbon rings 66 are stacked in the insulating tube 65 in such fashion that the rings can move up and down freely. The inside diameter of the carbon rings is large enough so that the rings will not make contact with the tubulation 46 which projects into the cavity within the stacked rings. The uppermost carbon ring makes contact with a metal ring 6! to which a terminal 68 is connected. At the lower end of the insulating tube 65 a copper terminal plate 69 fits inside the tube and makes contact with the lowermost carbon ring. The terminal plate 69 is held in place by a metal cup III, which in turn is urged upwardly by a helical spring 'II in firm contact with an expansion chamber 54, through which electrical contact is made to the bottom end of the tubulation 1i, and thence to a terminal clamp l1. Electric energy is supplied to the carbon pile heater from a suitable source (not shown) by the conductors II, 19. An orifice 12 in the casing 65 provides for the insertion of a thermocouple to measure the temperature of the tubulation and expansion chamber.

The expansion chamber or Sylphon 54 has deformable metal walls and contains a volatilizable substance as previously described in connection with Fig. 1. The tubulation 46 contains a reserve supply 15 of amalgam, or other suitable material capable of providing vapor for the discharge device. Such material may comprise cadmium amalgam.

In use, current from supply conductors 18, I9 flows through the carbon rings, its value being limited by the contact resistance of the rings. A metal cap is used to adjust the spring pressure on the stack of carbon rings until the desired current flows. Heat is generated electrically at the contact surfaces between the vari ous rings due to the relatively high transition resistance of.these contacts, and heats the tubulation 46 by radiation and conduction across the air gap.

The tubulation 46. heats the expansion chamber 54 by metallic conduction, causing it to buildup pressure by the vaporization of the charge 52 of ether or the like. The pressure thus generated acts against the spring H and lessens the compression on the carbon pile. This reduces the current through the carbon rings and lowers the rate of heating. When a balance is reached between the heat lost and the heat generated, the temperature remains substantially constant. The temperature of balance is insensitive to the rate of loss of heat, that is, to the external environment, because of the rapid change of pressure of the vaporizable substance with temperature. For example, a change of the ambient temperature from -40 C. to +40 C. changes the balance temperature by less than 2 C.

To change the temperature of the tubulation to a new value, adjustment is made by turnin the metal cap 80, which changes the spring pressure.

An arrangement such as that of Fig. 3 may be operated in an enclosure at a relative high temperature, and to this end the amalgam I5 may contain a, relatively small percentage of mercury and a relatively higher percentage of amalgamated material. The possibility of thus realizing high temperature operation obviates the need of cooling means which are expensive and troublesome, and enables the control of vapor pressure to be effected by heating only under all operating conditions.

By the regulating means indicated, it is readily possible .to hold the mercury vapor pressure within a discharge envelope to which the tubulation 46 is attached at a predetermined value within -1 C.

While the invention has been described by reference to a particular embodiment, it will be understood by those skilled in the art that numerous modifications may be made without departing from the invention. I therefore aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure. For example, the form of the various cooperating parts may be varied as illustrated by the variation of form of the expansion chambers shown in Figs. 4 and 5. The side walls 82 of the chamber of Fig. 4 are convolute, while the walls 83 of Fig. 5 are substantially spherical. The chamber of Fig. 5 may consist of a self-sealing rubber or plastic, such as neoprene, the charge 84 being :introduced by a needle, the puncture sealing itself when the needle is withdrawn. In the expansion device shown in Figfli, the end of the tubulation 48 which contains the amalgam 15, or other vapor-yielding material, projects into direct contact with the easily volatilizable material 85 which is present in suilicient quantity to surround the lower end 01 this tubulation. This arrangement provides for quick response of the pressure in the Sylphon chamber to the temperature of the vapor-yielding material.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric discharge device of the type which employs vapor of a material condensible at room temperature as a discharge-assisting medium, the combination which includes a discharge envelope enclosing cooperating electrodes, a hollow structure outside the main body of the envelope and connected by an unrestricted,

wide bore passage with the interior of the envelope, a quantity of vapor-yielding material located in the structure, a heater therefor, and means containing a volatile material the pressure of which is responsive to changes in the temperature 01' the said structure for regulating the amount of heat delivered to the structure to maintain the same at a relatively constant predetermined elevated temperature.

2. An electric discharge device employing mercury vapor as an ionizable medium comprising the combination of a main envelope containing discharge-supporting electrodes, an auxiliary container in unobstructed communication with said main envelope, a solid amalgam in said container for supplying mercury vapor, an electric heater therefor, and means responsive to departures of temperature outside of predetermined limits for energizing and deenergizing said heater to impart to said container heat in such amounts that said container is maintained within narrow predetermined temperature limits.

3. A discharge device of the type which employs mercury vapor as an ionizable medium, the combination which includes an envelope, electrode structure within the envelope, an auxiliary chamber connected by a wide bore opening with the space enclosed by the envelope and adapted to be maintained at a temperature below the temperature of the envelope, a quantity of solid amalgam located in the said chamber, said amalgam being effective to retain its mercury constituent in substantially nonvaporous condition during inoperative periods of the device and being further adapted to provide mercury vapor at a desired pressure during operative periods of the device, means for heating said chamber during operative periods of said device, and a thermostatic control means located in close thermal relation to said auxiliary chamber for controlling said heating means.

4. In a discharge device of the type which employs mercury vapor as an operating medium, the combination which includes an envelope containing electrode structure, means outside the envelope providing an auxiliary chamber communicating with the interior of the envelope, .9. quantity of amalgam of a metal of the class consisting of cadmium and tin within the said chamber, said amalgam being eflective to retain its mercury constituent in non-vaporous form within said chamber during inoperative periods of the device and being effective to supply mercury vapor at a desired pressure to the interior of the envelope during operative periods of the device, a Sylphon thermostat containing an easily vaporizable substance, and means responsive to the expansion and contraction of said thermostat for maintaining said amalgam at a predeterminedtemperature during operative periods of said device.

5. An electrical discharge device oi the type which employs mercury vapor as a dischargeassistingmedium, the combination which includes an envelope enclosing electrode structure, a hollow tubulation outside the main body 01' the envelope, but having a connection with the interior of the envelope, means for electrically heating said tubulation, temperature-controlling means for the said heating means, a quantity of cadmium amalgam located in the tubulation, a Sylphon thermostat containing a quantity of easily vaporizable material in good thermal relation to said tubulation and an electric regulator operable by said thermostat for energizing and deenergizing said heating means.

6. In a discharge device of the type which employs mercury vapor as a discharge-assisting medium, the combination which includes an envelope enclosing electrode structure, an auxiliary chamber connecting with the interior of the envelope and adapted to maintain a temperature independent of the temperature of the envelop, a quantity of cadmium amalgam within the said auxiliary chamber, said cadmium being eifective to retain its mercury constituent within the said condenser during inoperative periods of the device and to release the said mercury in vaporous form during operation of the device, an electric heater for said chamber, second chamber containing a quantity of ether in good thermal relation to said amalgam, and means responsive to the pressure in said second chamber for controlling the energy input of said heater.

7. An electrical discharge device comprising a sealed envelope consisting principally of metal, cooperating electrodes therein, a tubulatlon consistingprincipally of metal communicating with said envelope, a carbon pile resistor surrounding said tubulation, a movable cup bearing against said pile, a temperature-responsive expansion device containing a volatile substance and having deformable walls placed between said tubulation and said cup, a housing surrounding said cup and regulable means therein for exerting pressure upon said cup.

8. An electrical discharge device comprising the combination of a main envelope, cooperating electrodes therein, an auxiliary communicating chamber, means in said chamber for supplying mercury vapor, an electric heater for said chamber, an expansion device containing a substance having a materially higher vapor pres- ..sure than mercury in good thermal relation to --'said chamber. an electric means operable by said expans on device whereby the operations of said heater are controllable to maintain the pressure of mercury vapor in said main envelope within desired limits. 7

9. An electrical discharge device comprising the combination of a main envelope, cooperating electrodes therein including a thermionic cathode, an anode and a, discharge-controlling grid, an auxiliary communicating chamber, a mercury vapor-yielding charge therein, means for inductively heating said charge, a Sylphon heat-responsive device, means operative by predetermined movements of said Sylphon device for supplying varying amounts of energy to said inductive means to thereby maintain the pressure of mercury vapor in said main envelope within predetermined limits.

10, An electric discharge device comprising the combination of an envelope, cooperating electrodes therein, said envelope having a projecting tubulation providing an auxiliary chamber, means in said tubulation for providing mercury vapor, means for heating said tubulation, a heat-responsive Sylphon, device containing a volatilizable material which is in contact with said tubulation, and means for maintaining the temperature of said tabulation within predetermined limits in response to the vapor pressure in saidSylphon device.

11. An electrical discharge comprising the combination of a main envelope, cooperating electrodes therein, a hollow auxiliary metal tubulation connected by a wide communication unobstructed for the exchange of vapor with said main envelope, a quantity of solid amalgam in said tubulation, a carbon pile heater surrounding said tubulation, a Sylphon thermostat containing a quantity of ether which is in direct contact externally with said tabulation and means for regulating pressure on said carbon pile in response to the pressure of the ether vapor in said Sylphon.

ALBERT W. HULL. 

